Inbox displays new mail message but nothing new appears in Inbox

Hi.

I've got a some asking me for help because their Inbox shows the number 11
in bright blue next to its title which usually means the person has 11 new
unread mails. Thing is though when the go into the inbox to read these
message nothing new has turned up. I've checked that there not appearing in
subfolders in the inbox and that it's not old mail turning up further down
the list before more new e-mail that has been read. He's been able to
recieve individual messages from me that i sent that did come through top his
inbox and so we thought it was just an Outlook glitch but it has happened
again now today after we got rid of the number yesterday by just right
clicking on the inbox and selecting 'mark all as read'. He's a bit worried
now though and would like a fix so would appreciate any thoughts anyone has.

Thanks

Ash